The invention relates to a method for monitoring the capability of a catalytic converter to convert nitrogen monoxide into nitrogen dioxide in the exhaust gas of an internal combustion engine, the catalytic converter being arranged in an exhaust gas duct of the internal combustion engine and the catalytic converter being followed downstream by a collecting particle sensor.
Legal regulations require that internal combustion engines undergo a monitoring of all emission-relevant components. For instance, exhaust treatment components and the associated sensor equipment have to be monitored as part of an “On-Board Diagnosis” (OBD) for compliance with limit values, which are usually specified as a multiple of the emission limit values. A partial aspect of this is the monitoring of the coating of a catalyst, in particular of a diesel oxidation catalyst, for adequate capability of oxidizing nitrogen monoxide into nitrogen dioxide, and consequently of providing a suitable reactant (feed gas) for an SCR catalyst (SCR=Selective Catalytic Reaction), which is downstream in the exhaust gas duct and in which nitrogen oxides are converted by means of urea into carbon dioxide, nitrogen and water.
According to the prior art, in the monitoring of components as part of the on-board diagnosis, it is generally necessary to restrict the operating parameters under which the monitoring can be carried out. To improve the distinction between a component that only just has to be rejected (best part unacceptable) and a component that can only just be accepted (worst part acceptable), the plausibility functions are then only carried out for a restricted range of one or more of the following variables: exhaust mass flow, exhaust volume flow, exhaust temperature, rotational speed, quantity of fuel injected, vehicle speed, ambient pressure, ambient temperature or exhaust gas recirculation rate. Restrictions may also be provided for the operating mode, status, running time or service life of the internal combustion engine or for ranges of the signals for nitrogen oxide, hydrocarbon, carbon monoxide, particle mass or oxygen concentration. Some monitoring methods also envisage only carrying these out under steady-state or virtually steady-state operating conditions of the internal combustion engine.
Particle filters are often used in combination with a diesel oxidation catalyst arranged upstream in the exhaust gas duct of internal combustion engines to reduce the particle emission of diesel engines. The exhaust gas is passed through the particle filter, which separates the solid particles that are in the exhaust gas and keeps them back in a filter substrate. Over time, the particle filter becomes clogged with the masses of particulate matter that are lodged in the filter substrate, which becomes noticeable by an increase in the flow resistance, and consequently in the pressure opposing the exhaust gas. For this reason, the lodged mass of particulate matter must from time to time be discharged in a regeneration process.
Collecting particle sensors, which are usually arranged downstream of the particle filter, are known for monitoring the function of the particle filter. Particulates from the exhaust gas are deposited on the surface of the collecting particle sensors. Depending on the particle loading, the particle sensor sends a corresponding output signal, which allows conclusions to be drawn about the amount of particles carried in the exhaust gas, and consequently about the retention capability of the particle filter.
In the case of one embodiment of collecting particle sensors, at least two interengaging interdigital electrodes are provided on a substrate. Particulates deposited on them increasingly short the two electrodes, which allows a flow of current between the electrodes. In a possible method of evaluation for such a particle sensor, the variation in current when a voltage is constantly applied between the interdigital electrodes is evaluated. For this purpose, the time between the beginning of a measuring cycle and the reaching of a flow of current referred to as the triggering threshold is measured. The time it takes until the triggering threshold is reached is a measure of the particle content in the exhaust gas. After the measuring cycle, the particle sensor is heated up in a regeneration process and the deposited particles are burned, so that a new measuring cycle can subsequently be started.
Even today there is still no known procedure that is technically possible on a mass-production scale for checking catalytic converters for their capability of setting a composition of the exhaust gas that is suitable for a following exhaust treatment step, in particular an NO/NO2 ratio that is suitable for a selective catalytic reaction. It is therefore the object of the invention to provide a method that allows the monitoring of a catalytic coating for its capability of converting nitrogen monoxide into nitrogen dioxide.